When writing multipart data, this converter uses other
HttpMessageConverters to write the respective
MIME parts. By default, basic converters are registered (for Strings
and Resources). These can be overridden through the
partConverters property.

setCharset

Set the default character set to use for reading and writing form data when
the request or response Content-Type header does not explicitly specify it.

By default this is set to "UTF-8". As of 4.3, it will also be used as
the default charset for the conversion of text bodies in a multipart request.
In contrast to this, setMultipartCharset(java.nio.charset.Charset) only affects the encoding of
file names in a multipart request according to the encoded-word syntax.

map - the object to write to the output message. The type of this object must have previously been
passed to the canWrite method of this interface, which must have returned true.

contentType - the content type to use when writing. May be null to indicate that the
default content type of the converter must be used. If not null, this media type must have
previously been passed to the canWrite method of this interface, which must have
returned true.